This application claims the benefit of Korean Patent Application No. 2000-6225, filed on Feb. 10, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a reflective liquid crystal display device, and more particularly to a liquid crystal display device having a cholesteric liquid crystal color filter.
2. Discussion of the Related Art
LCD devices are usually classified into transmissive type and reflective type according to their difference in a light source.
The transmissive LCD device uses light incident from a back light that is attached to a rear surface of a liquid crystal panel. The light is incident to a liquid crystal layer of the liquid crystal panel, and is absorbed or passes through the liquid crystal layer according to different alignments of the liquid crystal layer. Therefore, dark or white mode is operated by the liquid crystal panel. Conventionally, the back light of the transmissive LCD device is an artificial light source. Therefore, high power consumption due to the back light is a greater disadvantage of the transmissive LCD device.
On the contrary to the above-mentioned transmissive LCD device, the reflective LCD device uses an ambient light incident from a natural light source or an exterior artificial light source. Because of its low power consumption, the reflective LCD device is focused on.
FIG. 1 is a cross-sectional view of a conventional reflective color LCD device.
As shown, upper substrate 13 includes a color filter 15 and a common electrode 17, and a lower substrate 11 includes a reflective electrode 16. Between the upper and lower substrates 13 and 11, a liquid crystal layer 19 is interposed between. Since the liquid crystal layer 19 has an optical anisotropy, molecules of the liquid crystal layer 19 are aligned in a proper direction with an electric field applied there-across. Therefore, an incident light to the liquid crystal layer 19 is controlled by the electric field applied across the liquid crystal layer 19 via the common and reflective electrodes 17 and 16. Instead of the liquid crystal layer 19, a certain medium having the optical anisotropy may be used for an LCD device.
On each exterior surface of the upper and lower substrates 13 and 11, a plurality of layers are formed to control a polarization of the incident light. For example, a retardation layer 23, and a polarizer 25 are sequentially formed on the exterior surface of the upper substrate 13. The retardation layer 23, or a quarter wave plate (QWP), makes an incident light right-circularly polarized (RCP) or left-circularly polarized (LCP). The polarizer 25 serves to pass only a portion of an incident light that corresponds to a transmittance axis direction of the polarizer 25. Other portions of the incident light that are different from the transmittance axis direction of the polarizer 25 are absorbed by the polarizer 25. At this point, a RCP ray is a right-circularly polarized ray that goes from a viewer, while an LCP ray is a left-circularly polarized ray that goes from the viewer.
A twisted nematic (TN) liquid crystal is typically used for the liquid crystal layer 19 between the upper and lower substrates 13 and 11. Long axes of the TN liquid crystal molecules are twisted to 90 degrees, and the TN liquid crystal layer is designed to have a phase difference (phase delay) of xe2x80x9cxcex/4xe2x80x9d.
Now, with reference to FIGS. 2A and 2B, xe2x80x9con and offxe2x80x9d states of the conventional reflective LCD device shown in FIG. 1 will be explained. FIG. 2A shows a passage of an incident light in the conventional reflective LCD device during its off state. At this point, a viewer (not shown) viewing the incident light is fixed at one position. A symbol xe2x80x9cxxe2x80x9d of ray denotes that the ray proceeds from the viewer, while another symbol xe2x80x9cxc2x7xe2x80x9d of ray denotes that the ray proceeds toward the viewer.
The incident light shown in FIG. 1 first meets the polarizer 25. As the light passes through the polarizer 25, it is linearly polarized. A first linearly polarized ray xe2x80x9cR1xe2x80x9d meets the retardation layer 23, passes through it, and becomes a first right-circularly polarized (RCP) ray xe2x80x9cR2xe2x80x9d. The RCP ray R2 further meets the liquid crystal layer 19, passes through it, and becomes a second linearly polarized light xe2x80x9cR3xe2x80x9d. Thereafter, the reflective electrode 16 reflects the second linearly polarized light R3 such that the second linearly polarized ray R3 change its direction and meets the liquid crystal layer 19 again. As a reflected ray R4 that has the opposite direction to the second linearly polarized ray R3 passes through the liquid crystal layer 19, the reflected ray R4 is right-circularly polarized with the phase difference of xcex/4. At this point, a second RCP ray xe2x80x9cR5xe2x80x9d is shown to rotate left and to proceed to the viewer. Then, the second RCP ray R5 meets the retardation layer 23, passes through it, and becomes a third linearly polarized ray xe2x80x9cR6xe2x80x9d. Since the third linearly polarized ray xe2x80x9cR6xe2x80x9d is parallel with the transmittance axis direction of the polarizer 25, it passes through the polarizer 25. Therefore, in the off state, the conventional reflective LCD device operates the white state.
Now, the TN liquid crystal layer 19 is on state such that molecules thereof arrange in one direction, for example perpendicular to the substrates 11 and 13, corresponding to an electric field applied across the TN liquid crystal layer 19. Therefore, the TN liquid crystal layer 19 is homeotropic-aligned.
In FIG. 2B, the incident light becomes the first RCP ray R 2 after it sequentially passes through the polarizer 25 and the retardation layer 23. This is the same as shown in FIG. 2A. Thereafter, since the TN liquid crystal layer 19 is on state, the first RCP ray R2 passes through the TN liquid crystal layer 19 without rotation and phase difference. Then, the reflective electrode 16 reflects the first RCP ray R2 such that it changes its direction to be a left-circular polarized (LCP) ray R7. The LCP ray R7 passes through the TN liquid crystal 19 that is still on state without rotation and phase difference, and meets the retardation layer 23. After the LCP ray R7 passes through the retardation layer 23, it is linearly polarized. At this point, a fourth linearly polarized ray R8 is perpendicular to the transmittance axis direction of the polarizer 25, and thus us absorbed by the polarizer 25. Accordingly, in its on state, the conventional reflective LCD device operates a dark state.
Rays passing through the color filter 15 have colors corresponding to the color of the color filter 15, for example red (R), green (G), and blue (B). When the conventional reflective LCD device is off state, a ray having a color passes through the polarizer 25 and displays the color. However, when the conventional reflective LCD device is on state, a ray having a color is absorbed by the polarizer 25 and does not display the color. Therefore, portions of the liquid crystal layer 19 respectively corresponding to colors of the color filter 15 are selectively on or off to display a color image.
However, in the above-mentioned conventional reflective LCD device, the incident light passes through too many layers. Whenever meeting new layers, or new mediums having different refractive indexes, each ray is somewhat reflected and absorbed by the new medium. Therefore, whenever passing through a new layer, the ray is weakened. For example, as the incident light passes through the polarizer 25, it is weakened for the first time. Further, as a ray passes through the reflective electrode 15, it is not only reflected but also absorbed and rapidly weakened.
In another aspect, color dispersion property and contrast ratios of the conventional reflective LCD device rapidly vary with respect to a viewing angle. That is to say, when a user looks at a color image displayed on the conventional reflective LCD device in a wide viewing angle, colors of the color image are shown shifted to different colors with a low contrast ratio.
Accordingly, the present invention is directed to a reflective LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An aspect of the present invention is to provide a reflective LCD device having low color dispersion and high contrast ratio in a wide range of a viewing angle.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.